Sintered silicon nitride products are excellent in various properties such as mechanical strength, heat and corrosion resistance, etc., and they are used as high temperature structural materials, such as for parts of gas turbines. However, silicon nitride has a poor sinterability because of its high covalent bond, and consequently, it is difficult to obtain sintered silicon nitride product having high density and high strength.
Hitherto, sintered silicon nitride products have been fabricated by (1) reaction sintering process which comprises molding and nitriding metallic silicon, (2) pressureless sintering process, and (3) hot-pressing process whose starting materials are silicon nitride powder containing sintering aids. However, these processes have advantages and disadvantages shown in the following Table 1.
TABLE 1 ______________________________________ Advantage Disadvantage ______________________________________ (1) Even complex Density is low, and Reaction sintered products strength, corrosion sintering can be fabricated. resistance, and process Degradation of oxidation resistance strength hardly are inferior. occurs, even at high temperatures. (2) It is possible to Deformation is easily Pressureless sinter even in the caused by shrinkage sintering case of a product during sintering. process having a compli- The largest amount of cated shape, but sintering aids is not as complex as required, and conse- in (1). quently degradation of strength is significant at high temperatures. Further, oxidation resistance is inferior. (3) Strength, oxida- Only simple shaped Hot-pressing tion resistance sintered products can process and corrosion be obtained. resistance are Degradation of excellent. strength at high temperature is less than in pressureless sintering process because of lesser sintering aids. ______________________________________
All of these processes have advantages and disadvantages. Therefore, none of them are completely satisfactory processes for fabricating high temperature structural materials having a complicated shape and good oxidation resistance, such as parts for gas turbines, engines, etc. However, since the pressureless sintering process and the hot-pressing process each has certain advantages, they would become desirable processes for fabricating the high temperature structural materials, if a method of overcoming the disadvantages while keeping the advantages of each could be found.